Clutch

ABSTRACT

The clutch is provided with a drive-side rotational body, a driven-side rotational body, driven-side rotational body, and an urging member. The driven-side rotational body is movable in the axial direction of the drive-side rotational body between a first position, at which the driven-side rotational body is coupled to the drive-side rotational body, and a second position, at which the driven-side rotational body is decoupled from the drive-side rotational body. The urging member urges the driven-side rotational body from the second position toward the first position. The driven-side rotational body includes a helical groove that extends in the urging direction of the urging member. The clutch is further provided with a pin that can be inserted into the helical groove.

TECHNICAL FIELD

The present disclosure relates to a clutch that switches power transmission states between a drive-side rotational body and a driven-side rotational body by selectively coupling and decoupling the rotational bodies to and from each other.

BACKGROUND ART

As proposed by Patent Document 1, a clutch may be arranged between a crankshaft and an auxiliary device to decrease friction related to an internal combustion engine. A clutch described in Patent Document 1 includes a drive-side rotational body and a driven-side rotational body. The drive-side rotational body is coupled to a crankshaft and thus rotated. The driven-side rotational body is coupled to an auxiliary device and rotational relative to the drive-side rotational body. The rotational bodies are pressed against each other by magnetic force produced by magnets. This maintains the clutch in an engaged state. The clutch is disengaged by energizing the coil arranged in the clutch to produce a magnetic field that cancels the aforementioned magnetic force.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-203406

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

As described in Patent Document 1, in the configuration in which the clutch is engaged by pressing the drive-side rotational body and the driven-side rotational body against each other, the force needed for such pressing becomes greater as the torque that needs to be transmitted through the clutch, or, in other words, the torque needed by the auxiliary device driven by the driven-side rotational body, becomes greater. To increase the pressing force, magnets with a greater magnetic force must be employed. This necessitates a larger-sized coil to cancel the magnetic force, and the clutch may become larger in size.

Accordingly, it is an objective of the present disclosure to provide a clutch capable of switching power transmission states without being enlarged in size to transmit greater torque.

Means for Solving the Problems

To achieve the foregoing objective and in accordance with one aspect of the present invention, a clutch is provided that includes a drive-side rotational body, a driven-side rotational body, an urging member, a helical groove, and a pin. The driven-side rotational body is movable in an axial direction of the drive-side rotational body between a first position at which the driven-side rotational body is coupled to the drive-side rotational body and a second position at which the driven-side rotational body is decoupled from the drive-side rotational body. The urging member urges the driven-side rotational body from the second position toward the first position. The helical groove is formed in the driven-side rotational body and extended in an urging direction of the urging member. The pin can be inserted into the helical groove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a clutch according to one embodiment;

FIG. 2 is a side view showing a sliding piece of the clutch illustrated in FIG. 1;

FIG. 3 is a perspective view showing the sliding piece shown in FIG. 2;

FIG. 4 is a perspective cross-sectional view showing a groove portion of the sliding piece of FIG. 3;

FIG. 5 is a perspective view showing the sliding piece with a holding device fixed to the sliding piece;

FIG. 6A is a diagram illustrating the clutch in an engaged state;

FIG. 6B is a diagram illustrating the clutch in a disengaged state;

FIG. 7 is a front view showing the sliding piece of FIG. 2, a solenoid, and a pin;

FIGS. 8A and 8B are cross-sectional views each showing the solenoid illustrated in FIG. 7; and

FIGS. 9A, 9B, 9C, 9D, 9E, and 9F are diagrams each illustrating operation of the clutch.

MODES FOR CARRYING OUT THE INVENTION

A clutch according to one embodiment will now be described with reference to FIGS. 1 to 9. The present embodiment is a clutch adapted for a vehicle, or, more specifically, a clutch arranged between the crankshaft of an engine and the compressor of an air conditioner to switch power transmission states between the crankshaft and the compressor.

As illustrated in FIG. 1, the clutch includes a pulley 10 and a sliding piece 20. The pulley 10 corresponds to a drive-side rotational body and the sliding piece 20 corresponds to a driven-side rotational body.

The pulley 10 is formed substantially in a conical shape having an outer diameter that increases toward the basal end of the pulley 10 (toward the right end as viewed in FIG. 1). A plurality of grooves 11, over which a belt is looped, are formed in the outer circumferential surface of the basal end portion of the pulley 10. The belt is also looped over the crankshaft and thus allows synchronous rotation of the crankshaft and the pulley 10. A projection 12, which axially projects from the inner circumferential surface of the pulley 10, is formed on the inner circumferential surface of the pulley 10. The projection 12 has an annular shape extending about the axis of the pulley 10. The pulley 10 is supported by a compressor shaft 70 of a compressor through a bearing 13, which is arranged at the distal end of the pulley 10 (the left end as viewed in FIG. 1), in a manner rotational relative to the compressor shaft 70.

The compressor shaft 70 is supported by an engine body 72 through a bearing 71 in a manner rotational relative to the engine body 72. When the compressor shaft 70 rotates, the compressor is driven to compress refrigerant. An annular plate assembly 73 is fixed to the surface of the engine body 72 that faces the pulley 10.

A straight spline 74 is formed in the outer circumferential surface of the portion of the compressor shaft 70 between the bearings 13 and 17. The sliding piece 20 is meshed with the straight spline 74. This allows the sliding piece 20 to rotate integrally with the compressor shaft 70 and move axially relative to the compressor shaft 70. The sliding piece 20 is urged by a coil spring 21, which is arranged between the sliding piece 20 and the engine body 72, toward the pulley 10 (toward the left end as viewed in FIG. 1). The spring 21 corresponds to an urging member. A plurality of balls 22 are provided in the outer circumferential surface of the sliding piece 20. The diameter Db of each of the balls 22 is substantially equal to the distance from the outer circumferential surface of the sliding piece 20 to the projection 12 of the pulley 10. The balls 22 thus contact the outer circumferential surface of the sliding piece 20 and the inner circumferential surface of the projection 12. A holding device 23, which restricts movement of the balls 22, is fixed to the outer circumferential surface of the sliding piece 20. The pulley 10, the compressor shaft 70, and the sliding piece 20 are arranged coaxially. Hereinafter, the extending direction of the axis of these components will be referred to as the axial direction.

The configuration of the sliding piece 20 will hereafter be described with reference to FIGS. 2 to 5.

With reference to FIGS. 2 and 3, the sliding piece 20 includes a holding portion 24, which holds the balls 22 at the outer circumferential surface and a groove portion 25, which is arranged close to the engine body 72 (on the right side as viewed in FIG. 2) with respect to the holding portion 24.

The holding portion 24 has a pillar portion 26 having a substantially circular cross section and a hexagonal column portion 27 having a regular hexagonal cross section. The hexagonal column portion 27 has six sides 28 and six corners 29. The diameter of the imaginary circle including midpoints 30 of the sides 28 is equal to the outer diameter of an end surface 31 of the pillar portion 26. In contrast, the diameter of the imaginary circle including the corners 29 is greater than the outer diameter of the end surface 31 of the pillar portion 26. The cross section of the pillar portion 26 is thus varied gradually from the circular shape to the hexagonal shape toward the hexagonal column portion 27.

A hole 32, which extends in the axial direction and is meshed with the straight spline 74 of the compressor shaft 70, is formed in a central portion of the holding portion 24.

The configuration of the groove portion 25 of the sliding piece 20 will now be described.

As illustrated in FIGS. 2 and 4, the groove portion 25 has a helical groove 33 and an annular groove 34. The helical groove 33 extends in a manner revolving by 540° clockwise about the axis of the sliding piece 20 from a starting point 35 (0°), which is the uppermost point of the helical groove 33 as viewed in FIGS. 2 to 4. The diameter of the helical groove 33 becomes smaller in a direction circumferentially separating from the starting point 35. That is, an extended surface of the helical groove 33 facing the hexagonal column portion 27 of the holding portion 24, which is a side surface (or a side wall) 36, becomes closer to the pulley 10 in the direction circumferentially separating from the starting point 35. In the helical groove 33, the radially inner edge of a first-cycle side surface 361 is connected to the radially outer edge of a second-cycle side surface 362 via a circumferential surface 37. As a result, as shown in FIG. 2, the helical groove 33 has a stepped cross section.

In the present embodiment, the direction in which the helical groove 33 converges toward the axis of the sliding piece 20 is the extending direction of the helical groove 33. That is, a direction axially extending toward the pulley 10, which is, in other words, the direction in which the spring 21 urges the sliding piece 20, is the extending direction of the helical groove 33.

The annular groove 34, which extends circumferentially about the axis of the sliding piece 20, is arranged at a position forward (to the left as viewed in FIG. 2) of the helical groove 33 in the urging direction of the spring 21 and extended along the entire circumference of the sliding piece 20. The helical groove 33 has an ending point 38, which is located at the position revolved by 540° from the starting point 35. The annular groove 34 extends continuously from the helical groove 33 and starts from a starting point 40, which is the ending point 38 of the helical groove 33. The annular groove 34 is formed such that an extended surface, or a side surface (or a side wall) 39, of the annular groove 34 extends substantially at a constant axial position. The radial length of the side surface 39 becomes gradually smaller in a direction circumferentially separating from the starting point 40 of the side surface 39. Accordingly, the cycle of revolution (from 540° to 900°) along which the annular groove 34 extends ends at the ending point 38 of the helical groove 33.

The configuration of the holding device 23, which is fixed to the outer circumferential surface of the sliding piece 20, will hereafter be described with reference to FIG. 5.

Referring to FIG. 5, the holding device 23 is shaped like a tube and includes an inner hole 41 having a shape substantially identical with the cross section of the hexagonal column portion 27 of the holding portion 24, which is the regular hexagonal shape. The holding portion 24 of the sliding piece 20 is inserted into the inner hole 41 to fix the holding device 23 to the hexagonal column portion 27 of the sliding piece 20. The holding device 23 thus rotates integrally with the sliding piece 20. Holding holes 42, each of which extends about the associated one of the midpoints 30 of the holding portion 24, are formed in the respective portions of the holding device 23 corresponding to the midpoints 30. Each holding hole 42 is shaped such that the circumferential length of the holding hole 42 becomes smaller in a direction approaching the pillar portion 26 of the holding portion 24. The balls 22 are arranged in the associated holding holes 42 and thus maintained at predetermined positions on the outer circumferential surface of the sliding piece 20. The balls 22 are permitted to move to the outer circumferential surface of the pillar portion 26 and the outer circumferential surface of the hexagonal column portion 27. Also, when located on the outer circumferential surface of the hexagonal column portion 27, the balls 22 are permitted to move circumferentially in a predetermined range.

A manner in which the clutch having the above-described configuration switches power transmission states will hereafter be described.

FIG. 6A illustrates the balls 22 each in a state located on the outer circumference of the hexagonal column portion 27 of the holding portion 24 of the sliding piece 20. From the midpoint 30 of each side 28 toward the associated corner 29, the clearance between the outer surface of each of the sides 28 of the hexagonal column portion 27 and the projection 12 of the pulley 10 becomes smaller with respect to the diameter Db of each ball 22. Accordingly, as the pulley 10 rotates clockwise as viewed in FIG. 6A, for example, the balls 22 contacting the inner circumferential surface of the projection 12 are moved clockwise and caught in the portions corresponding to the smaller clearance to be in a non-rotational state. This couples the pulley 10 and the sliding piece 20 to each other, or, in other words, engages the clutch. The rotational force of the pulley 10 is thus transmitted to the sliding piece 20. This rotates the compressor shaft 70, switching the compressor to an operating state.

In contrast, when the balls 22 are located on the outer circumference of the pillar portion 26 of the sliding piece 20 as illustrated in FIG. 6B, the clearance between the outer circumferential surface of the pillar portion 26 and the projection 12 is maintained substantially equal to the diameter Db of each ball 22 throughout the entire circumference. Accordingly, when the pulley 10 rotates clockwise as viewed in FIG. 6B in this state, for example, the balls 22 are permitted to rotate between the sliding piece 20 and the projection 12. The pulley 10 and the sliding piece 20 are thus in a decoupled state, or, in other words, the clutch is disengaged. This prevents transmission of the rotational force of the pulley 10 to the sliding piece 20. Rotation of the compressor shaft 70 is thus suspended, and the compressor is in a non-operating state.

As shown in FIG. 7, the plate assembly 73 has a solenoid 50, which is arranged radially outside of the sliding piece 20, and a pin 60 coupled to the solenoid 50. The pin 60 has an elongated hole 61 formed in an end and an inserting portion 62, which is formed in the opposite end of the pin 60 and can be inserted into the groove portion 25 of the sliding piece 20. The plate assembly 73 also has a shaft 63, which is arranged between the end portions of the pin 60 to support the pin 60 in a manner rotational and immovable in the axial direction of the pulley 10. In FIG. 1, illustration of the solenoid 50 and the pin 60 is omitted.

A movable iron core 51 of the solenoid 50 is connected to the elongated hole 61 of the pin 60 through a coupling member 64. When the movable iron core 51 is projected from the solenoid 50 as represented by the corresponding solid lines in FIG. 7, the pin 60 is pivoted about the shaft 63 to insert the inserting portion 62 of the pin 60 into the groove portion 25. In contrast, when the movable iron core 51 is retracted into the solenoid 50 as represented by the long dashed double-short dashed line in FIG. 7, the pin 60 is pivoted about the shaft portion 63 to retract the inserting portion 62 from the groove portion 25.

The configuration of the solenoid 50 will hereafter be described.

With reference to FIG. 8A, the solenoid 50 has the movable iron core 51, a yoke 52 holding the movable iron core 51, a coil 53 arranged in the yoke 52, a receiving portion (a fixed iron core) 58, and a magnet 54. A hole 55 is formed in a distal end portion of the movable iron core 51. The aforementioned coupling member 64 is attached to the hole 55 and inserted into the elongated hole 61 of the pin 60 (see FIG. 7). A ring member 56 is attached to the outer circumference of the movable iron core 51. A spring 57 is provided between the ring member 56 and the yoke 52. The spring 57 produces urging force acting in a direction in which the movable iron core 51 is projected from the yoke 52 (in the rightward direction as viewed in FIG. 8A).

When the movable iron core 51 is in a projected state as illustrated in FIG. 8A and the coil 53 is energized, the magnetic force produced by the coil 53 retracts the movable iron core 51 into the yoke 52. Then, when the movable iron core 51 comes into contact with the receiving portion 58, the attracting force (the magnetic force) by which the magnet 54 attracts the movable iron core 51 exceeds the elastic force of the spring 57. If the coil 53 is de-energized in this state, the attracting force of the magnet 54 exceeding the elastic force of the spring 57 maintains the movable iron core 51 in a retracted state, as illustrated in FIG. 8B.

In contrast, if the coil 53 is energized with the movable iron core 51 held in the retracted state such that a reverse field is produced to cancel the magnetic force of the magnet 54, the attracting force of the magnet 54 decreases and the elastic force of the spring 57 exceeds the attracting force. This projects the movable iron core 51 to such a position where the ring member 56 contacts the yoke 52. When the movable iron core 51 is in a state projected and separated from the receiving portion 58 in this manner, the elastic force of the spring 57 exceeds the attracting force of the magnet 54. Accordingly, once the coil 53 is energized to separate the movable iron core 51 from the receiving portion 58, the elastic force of the spring 57 maintains the movable iron core 51 in the projected state as illustrated in FIG. 8A, even after the coil 53 is de-energized.

Accordingly, by energizing the coil 53 of the solenoid 50, the movable iron core 51 is moved to switch the pin 60 selectively between an inserted state, in which the pin 60 is inserted into the helical groove 33, and a retracted state, in which the pin 60 is retracted from the helical groove 33. Further, even after the coil 53 is de-energized, the pin 60 is maintained in a state to which the pin 60 has been switched.

Operation of the clutch according to the present embodiment will now be described with reference to FIG. 9.

FIG. 9A illustrates a state in which the sliding piece 20 is urged by the urging force of the spring 21 and closest to the pulley 10 in the axial direction. Referring to the drawing, when the sliding piece 20 is closest to the pulley 10, the balls 22 are located on the outer circumference of the hexagonal column portion 27 of the sliding piece 20. If the pulley 10 rotates in this state, the balls 22 are caught between the projection 12 of the pulley 10 and the sliding piece 20 in a non-rotational state, thus coupling the pulley 10 and the sliding piece 20 to each other in a manner permitting power transmission. This switches the clutch to an engaged state such that the sliding piece 20 and the compressor shaft 70 rotate integrally with the pulley 10 to operate the compressor. The position at which the sliding piece 20 is located when the clutch engaged corresponds to a first position.

If the coil 53 is energized in this state, the inserting portion 62 of the pin 60 is inserted into the helical groove 33 of the sliding piece 20, which is in a rotating state, as illustrated in FIG. 9B. This causes the pin 60 to slide on the side surface 36 of the helical groove 33 to convert some of the rotational force of the sliding piece 20 into axial force. As a result, as illustrated in FIG. 9C, the sliding piece 20 is moved rightward in the axial direction, or, in other words, in the direction separating from the pulley 10. At this stage, as the sliding piece 20 moves in the axial direction, the balls 22, which are arranged in the outer circumference of the sliding piece 20, move from the positions on the outer circumferential surface of the hexagonal column portion 27 to the positions on the outer circumferential surface of the pillar portion 26.

Then, referring to FIG. 9D, as the pin 60 slides on the side surface 36 of the helical groove 33 and the axial movement amount of the sliding piece 20 increases, the balls 22 are guided by the corresponding holding holes 42 of the holding device 23 and moved to the outer circumference of the pillar portion 26. When the balls 22 are located on the outer circumference of the pillar portion 26, the balls 22 are in a state permitted to rotate between the sliding piece 20 and the projection 12. The sliding piece 20 and the projection 12 are thus decoupled from each other. This switches the clutch to a disengaged state, thus stopping operation of the compressor. The position at which the sliding piece 20 is located when the clutch is disengaged corresponds to a second position.

Even after the pulley 10 and the sliding piece 20 are decoupled from each other, the sliding piece 20 may be maintained in a rotating state by inertial force in some cases. However, if the pin 60 slides on the side surface 39 of the annular groove 34 as illustrated in FIG. 9D, the rotational force of the sliding piece 20 is prevented from being converted into the axial force. This restricts axial movement of the sliding piece 20 after the sliding piece 20 and the pulley 10 are decupled from each other.

To switch the clutch from the disengaged state back to the engaged state, the coil 53 is energized to retract the inserting portion 62 of the pin 60 from the annular groove 34, as illustrated in FIG. 9E. After the pin 60 is retracted from the annular groove 34, the sliding piece 20 is pressed by the urging force of the spring 21. This moves the sliding piece 20 leftward in the axial direction, which is the direction approaching the pulley 10. Such movement of the sliding piece 20 moves the balls 22 from the positions on the outer circumferential surface of the pillar portion 26 to the positions on the outer circumferential surface of the hexagonal column portion 27. Then, when the sliding piece 20 is pressed to such a position where the sliding piece 20 becomes closest to the pulley 10 referring to FIG. 9F, the balls 22 are arranged on the outer circumference of the hexagonal column portion 27. As a result, the balls 22 are caught between the pulley 10 and the sliding piece 20, thus engaging the clutch.

The above described embodiment has the following advantages.

(1) In the present embodiment, to disengage the clutch, the pin 60 is inserted into the helical groove 33 of the sliding piece 20, which is in a rotating state. This converts some of the rotational force of the sliding piece 20 into the axial force, which moves the sliding piece 20 to the second position. The force necessary for disengaging the clutch is thus generated from the rotational force of the sliding piece 20. As a result, even to transmit relatively great torque, the clutch does not have to be enlarged in size to be capable of switching power transmission states.

(2) In the present embodiment, the helical groove 33 is formed in a stepped shape as viewed along an axial cross section. The axial length of the portion corresponding to the helical groove 33 is thus decreased, and size enlargement of the clutch is restrained in a more desirable manner.

(3) In the present embodiment, the pin 60 is caused to slide on the annular groove 34 to restrict axial movement of the sliding piece 20 after the pulley 10 and the sliding piece 20 are decoupled from each other. The axial length of the clutch thus can be decreased to reduce the size of the space needed to install the clutch.

(4) In the present embodiment, the clutch includes the balls 22. When the sliding piece 20 is located at the first position, the balls 22 are caught between the sliding piece 20 and the pulley 10 in a non-rotational manner to couple the sliding piece 20 to the pulley 10. When the sliding piece 20 is arranged at the second position, the balls 22 are released from the sliding piece 20 and the pulley 10 to decouple the sliding piece 20 and the pulley 10 from each other. As a result, unlike a pressing type clutch, for example, the clutch is capable of transmitting greater torque without being enlarged in size.

(5) In the present embodiment, a self-holding type solenoid 50 is employed as the solenoid 50. Thus, the coil 53 needs to be energized only when the power transmission states of the clutch must be switched. This decreases the power consumed by the solenoid.

Although the self-holding type solenoid 50 is used as the solenoid 50 in the above illustrated embodiments, a solenoid that inserts the pin 60 into the helical groove 33 only while being energized may be employed. In this configuration, the clutch is disengaged only when the coil 53 is energized. Therefore, if the solenoid 50 fails to energize the coil, the clutch is maintained in the engaged state. As a result, the compressor is operated regardless of such failure of the solenoid 50.

In the above illustrated embodiment, the solenoid switches the pin 60 selectively between the inserted state and the retracted state. However, the states of the pin 60 may be switched using any suitable actuator other than the solenoid, such as a hydraulic type actuator.

In the above illustrated embodiment, the sliding piece 20 and the pulley 10 are selectively coupled to and decoupled from each other by means of the balls 22. However, a pressing type clutch having a pressing surface formed in a portion of the outer circumferential surface of the sliding piece 20 may be employed. The clutch switches to the engaged state by pressing the pressing surface against the pulley 10.

In the above illustrated embodiment, the groove portion 25 is formed in the outer circumferential surface of the sliding piece 20. However, the groove portion 25 may be formed in the inner circumferential surface of the sliding piece 20.

Although the above illustrated embodiment employs the helical groove 33 having the stepped cross section the diameter of which decreases in the axial direction toward the pulley 10, such configuration may be modified.

Although the above illustrated embodiment includes the annular groove 34, the annular groove 34 may be omitted.

In the above illustrated embodiment, the helical groove is shaped to extend in a manner revolving clockwise by 540° from the starting point 35 (0°). However, the range in which the helical groove 33 extends in a revolving manner may be set to any other suitable range, such as the range corresponding to 180° or 720°. That is, any suitable range may be employed as long as the sliding piece 20 can be moved to the second position by the pin 60 sliding on the helical groove 33.

Although the coil spring 21 is used as the urging member in the above illustrated embodiment, any other suitable member such as a spring or a rubber member having any other shape than the coil-like shape may be employed as the urging member.

In the above illustrated embodiment, the clutch is arranged between the crankshaft and the compressor to switch the power transmission states between the crankshaft and the compressor. However, the clutch according to the present disclosure may be used as a clutch provided between any other suitable auxiliary device, such as a water pump or an oil pump, and the crankshaft. Further, the clutch according to the present disclosure is not restricted to a clutch for switching states of power transmission from a crankshaft but may be employed as a clutch for switching states of power transmission from any other suitable drive source.

In the above illustrated embodiment, the axial position of the pin 60 is restricted. However, as long as the sliding piece 20 can be moved to the second position by engaging the pin 60 with the groove portion 25, the pin 60 may be allowed to move in the axial direction. 

1. A clutch comprising: a drive-side rotational body; a driven-side rotational body movable in an axial direction of the drive-side rotational body between a first position at which the driven-side rotational body is coupled to the drive-side rotational body and a second position at which the driven-side rotational body is decoupled from the drive-side rotational body; an urging member for urging the driven-side rotational body from the second position toward the first position; a helical groove formed in the driven-side rotational body and extended in an urging direction of the urging member; and a pin that can be inserted into the helical groove.
 2. The clutch according to claim 1, wherein the pin is immovable in the axial direction of the drive-side rotational body.
 3. The clutch according to claim 1, wherein the helical groove is formed in an outer circumferential surface of the drive-side rotational body and has a stepped shape having a diameter becoming smaller forward in the urging direction.
 4. The clutch according to claim 1, wherein the driven-side rotational body has an annular groove arranged forward of the helical groove in the urging direction and extended in a circumferential direction of the driven-side rotational body, the annular groove being connected to the helical groove, and the annular groove allows the pin to slide on the annular groove when the driven-side rotational body is arranged at the second position.
 5. The clutch according to claim 1, further comprising a ball through which the driven-side rotational body is coupled to the drive-side rotational body, wherein the ball is non-rotationally caught between the driven-side rotational body and the drive-side rotational body to couple the two rotational bodies to each other when the driven-side rotational body is located at the first position, and the ball is released from the driven-side rotational body and the drive-side rotational body to decouple the rotational bodies from each other when the driven-side rotational body is arranged at the second position.
 6. The clutch according to claim 1, further comprising a solenoid that switches the pin selectively between an inserted state in which the pin is inserted into the helical groove and a retracted state in which the pin is retracted from the helical groove, wherein the solenoid has a coil, and the solenoid switches the pin from one of the inserted state and the retracted state to the other when the coil is energized and maintains the pin in a state to which the pin has been switched when the coil is de-energized. 